When performing a heat treatment on a stepped workpiece having a relatively thick portion and a relatively thin portion provided on one or both sides of the thick portion, the workpiece is typically heated in a furnace from the viewpoint of uniform heating, but induction heating methods have also been proposed (see, e.g., Patent document 1).
An induction heating coil disclosed in Patent document 1 is configured to inductively heat a double-side-stepped workpiece having a large-diameter shaft portion and small-diameter shaft portions provided on both axial sides of the large-diameter shaft portion. This induction heating coil is configured such that a conductor extends parallel with an outer line of a cross section, including its center axis, of a workpiece, and generates a magnetic flux when supplied with high-frequency power. The workpiece is inductively heated receiving the magnetic flux generated by the induction heating coil while being rotated about its center axis.